This invention is in the field of rotary mowers and in particular is concerned with attaching a blade to such a mower.
A typical rotary mower includes a rotating blade pan with a plurality of mower blades pivotally attached to the pan about substantially vertical pivot axes located near outer edges of the pan. The pan rotates at high speed, and centrifugal force causes the blades to extend radially from the rotational axis of the pan, and past the edge of the pan to cut vegetation. This design reduces damage when the blades contact obstructions such as rocks or the like, since when a blade contacts an obstruction it will pivot backwards about its pivot axis until the obstruction is cleared, and then move out again in response to the centrifugal force of the rotating pan. The pan itself is substantially smooth on the bottom, and so will simply ride over any obstruction.
The blades are typically attached to the pan with a blade bolt. As described in Canadian Patent Application Number 2,477,180 of Toth, these blade bolts typically have a head, then a shoulder portion that bears against the bottom of the pan, then a shank portion that extends through a hole in the pan, and finally a threaded portion on which a nut is threaded to tighten the bolt. The bolt is inserted through a hole in the blade, and then into the hole in the pan. The shoulder provides a spacer so that the blade is not pinched between the head and the pan, but rather can pivot freely on the shoulder. Corners on the head of the bolt catch on vegetation under the pan, and so the head is typically circular to reduce the amount of vegetation that catches on the bolt head.
Because the head is circular and difficult to grasp with a wrench, it is desirable to provide a means to prevent the bolt from rotating with respect to the pan. This facilitates tightening the nut when installing the blade, and reduces the risk that the bolt will turn in the nut and separate from the nut and fall off the pan. As described by Toth, the bolt thus typically includes a key, or is square, hexagonal, pentagonal, or the like, such that the shank engages a corresponding aperture in the pan. Toth also describes the stress concentrations at the corners of the shank, especially a square shank, which can cause the pan to crack and break. The corners of hexagonal bolts can strip. Toth proposes a pentagonal shank as a compromise between the square shank which is prone to cracking, and the hexagonal shank which is prone to stripping, however the pentagonal shank with its five corners is only a compromise, and is still subject to both the cracking and stripping problems.